1. Field
Embodiments of the present invention relate to a complementary metal-oxide semiconductor (CMOS) image sensor (CIS) and, more particularly, to a reference voltage generator and a CIS using the same, which are capable of preventing pixel signals from being distorted due to power noise (for example, ground noise and supply noise) by cancelling or suppressing the power noise in the readout path of the CIS.
2. Description of the Related Art
In a typical column parallel structure, each comparator of a readout circuit operates in a differential mode, and thus, noise occurring due to a comparator supply voltage may be effectively cancelled in a common mode.
However, since power noise occurring in a pixel is applied to a readout circuit and included in a pixel signal, the power noise is not cancelled-out in the readout circuit and is outputted in the pixel data.
The power noise of the pixel may refer to supply noise or ground noise, or may be a combination of supply noise and ground noise through a specific circuit. More comprehensively, the power noise may be any type of power noise occurring in the pixels.
Since the power noise of the pixel is a major factor which generates horizontal noise in a CMOS image sensor (CIS), it is necessary to suppress or cancel the power noise of the pixel as much as possible.
In order to suppress or cancel the power noise of the pixel, a pixel signal and a reference signal (reference voltage) are to be transferred to a plus input terminal and a minus input terminal of a comparator, based on the same power supply.
A pixel supply voltage (VDDPX first power supply voltage) supplied to a pixel which outputs a pixel signal may be supplied to a reference voltage generator which generates a reference signal to transfer the pixel signal and the reference signal to two input terminals of the comparator based on the same power supply. At this time, the power may fluctuate due to a switching operation of a switching operation unit of the reference voltage generator, causing switching noise. The switching noise may be transferred to the readout circuit and be included in the reference signal without being filtered. Such noise increases horizontal noise in the CMOS image sensor.